[Coco] again: CoCo RGB to VGA conversion
nick at launch.net.au
Wed Aug 18 17:08:49 EDT 2004
>Isn't 640x192 (or whatever it is) going to look blocky ...
>cauz it is! I suspect the reason that a CrapMaster-8 does not look
>blocky is it does not have sufficient sharpness to reveal the "true
>blockiness" that is inherent in 640x192.
>Anyway I'll take blockiness as a tradeoff for not having to
>have a special monitor for the tre.
Yes, it will always be blocky which is why I said that the only way to get
it sharp is to increase the resolution of the CoCo to produce at least a
true res (not scan doubled) to 640x400. Of course the GIME can't produce
this so we're stuck with an output resolution of 640 x 200. Now to simply
scan double to create 640 x 400 video scanlines simply takes the 640x200 and
makes the pixels double-height ( multiplying the vertical resolution by 2).
This is what I mean by making it look even more blocky. Also, because a VGA
monitor is sharper, a pixel will have a distinct squareness to it unlike the
lower resolution monitor which still creates a reasonably round pixel (plus
blurring) with the blank gaps in between.
There are 2 tricks that may be possible:
The CoCo to VGA converter has to do more than scan double, it has to fill
the blank scanlines with valid data. This was the idea behind when I said to
create a device that captures 1 entire CoCo frame, keeps it in a buffer then
displays the next frame to the VGA but using scanlines from the buffered
image to fill the blank scanlines, this giving a full 640x400 resolution (in
other words a true VGA). Unfortunately, this circuit would be very complex
and expensive so it will never be a viable project.
The other idea is a de-interlacer. Sockmaster has found that using the 1986
version of the GIME, it can be fooled to do interlace video (640x400
interlaced). A circuit that de-interlaces to a standard VGA monitor could do
this. Unfortunately, the 1986 GIME also has a few more bugs that were
corrected in the 1987 version and I suspect most people are using the 1987
I run the MESS emulator with the scanlines option on (it simulates the blank
scanlines to make it look like a real CoCo on a CM8 type monitor). I find it
looks much better, not sharper, but less blocky that when run without
scanline (scandoubling). Graphics with scandoubling starts creating the
appearance of a much lower resolution because of the emphasis of square
Anyway, that's my spin on it! :)
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